The observed expansion of the Universe implies that it was born out of a singularity, a point of infinite density. How could anything that dense ever expand? Its enormously strong gravitational field should turn it into a black hole.
The maximum distance that light can have travelled since the Big Bang is called the horizon distance. Parts of the Universe are separated by many times this distance because, according to general relativity, space itself can expand faster than the speed of light. Nothing carrying energy can travel faster than the speed of light, yet regions visible from Earth but beyond each other’s horizon have almost the same cosmic background temperature. Simple Big Bang theory does not tell us how this happened.
The mean density of matter across the Universe is not very well known, but some determinations suggest that it is close to the critical value. But it is unlikely to be anywhere near this value unless it started out at exactly the critical density. This difficulty with density is sometimes called the flatness problem simply because a universe with critical density is flat.
In 1927 Werner Karl Heisenberg (1901-76) published his uncertainty principle, which puts strict limits on what it is possible to know about particles. In particular, it is impossible to know the exact energy content of a region of space over a given period of time. There is no such thing as a perfect vacuum; even the apparently empty regions of space are full of particles and antiparticles which randomly appear literally out of nowhere and then vanish, their short existence being detected by their effect on atoms. The reality of quantum vacuum energy has been verified in the laboratory.
In 1979 Alan Harvey Guth (1947- ) proposed that during a very short period after time zero, vacuum energy acting like antigravity accelerated the expansion of the Universe from a size smaller than a photon to about that of a golf ball. This brief period is known as the inflationary phase, and the theory is called the inflationary model. When the inflationary period ended, the Universe continued to expand but at the comparatively slower rate of the standard Big Bang model. This early period of exponential expansion of space-time is described by the de Sitter model.
The inflationary model offers reasonable solutions to the flatness and horizon problems. During the inflationary period any imbalance between the expansive force of the Big Bang and the contracting force of gravity would have been destroyed, leaving the Universe in a state of flatness. By the end of the inflationary period the Universe would still have been small enough for all its parts to be within each other’s horizon distance, i.e. there would have been enough time for all the parts to swap heat and come to the same temperature.
After the expansion of the Universe was discovered, Einstein began to regard his introduction of the cosmological constant as a mistake. Many cosmologists, however, now regard the energy released during the inflationary phase acts in much the same way as a cosmological constant.
For clouds of gas to clump together under their own gravity and collapse to form galaxies and stars, the early Universe is required to have had irregularities. But the present uniformity of the background radiation implies that at the end of the photon epoch 300,000 years after time zero the Universe was smooth. In April 1992, George Fitzgerald Smoot III (1945- ) announced that he and his colleagues had found fluctuations, or ‘wrinkles’ in the background radiation. The fluctuations measured no more than 30×10-6K but this was thought to be enough to explain what had happened to the Universe. It is possible that our region of space-time is not unique. It could be part of a larger region of space-time in which inflation could be producing other universes like our own. Andrei Dmitriyevich Linde (1948- ) called this chaotic inflation (i.e. complicated, not the chaos theory). This implies that inflation could occur in other parts of our universe in a process that would have had no beginning and no end.
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